The present invention relates to torque transmitting fluid couplings, and more particularly, to such couplings which utilize internal valving, whereby the fluid coupling may be in either an engaged or disengaged condition, depending upon the position of the valving.
Fluid couplings of the type to which the present invention relates are well known in the art and may be better understood by reference to U.S. Pat. Nos. 3,055,473; 3,174,600; and 3,339,689, all of which are assigned to the assignee of the present invention. Briefly, such fluid couplings typically include an output coupling member and a cover which cooperate to define a fluid chamber, a valve plate dividing the fluid chamber into an operating chamber and a reservoir chamber, and an input coupling member disposed within the operating chamber and rotatable relative to the output coupling member. The input and output coupling members define a shear space such that rotation of the input member causes viscous fluid in the shear space to exert a viscous drag on the output member, causing it to rotate. The valve plate defines a fill orifice, and a valving arrangement controls the flow of fluid from the reservoir chamber, through the fill orifice, into the operating chamber. Typically, the valving is temperature-responsive, as is illustrated in the above-cited patents, such that below a certain ambient temperature, the valving is closed, most of the viscous fluid is discharged from the operating chamber to the reservoir chamber and the fluid coupling is considered to be "disengaged". Above the predetermined temperature, the valving gradually opens and viscous fluid is permitted to flow from the reservoir into the operating chamber, filling the shear space, such that the coupling is "engaged".
Conventional fluid couplings of the type to which the present invention relates have been provided with relatively small clearances between the outer periphery of the input member and the inner periphery of the output member, partly because the viscous fluid between these adjacent peripheries acts as a fluid bearing, and partly to maximize the available shear surface and the torque transmitting capacity. Therefore, although the present invention may be utilized in fluid coupling devices of many different embodiments, it is especially useful in those of the type in which the outer periphery of the input member and the inner periphery of the output member have been closely spaced apart. It is also especially useful in those in which some form of valving is provided to control the flow of fluid into the operating chamber, such that the coupling may be utilized in either an engaged or a disengaged condition.
Conventional fluid couplings have generally been of the type referred to as "full OD", i.e., the outer surface of the input member and the inner surface of the output member are cylindrical and have a maximum diameter over the entire axial extent of the respective surfaces. As noted previously, a full OD input member provides maximum torque transmission when the fluid coupling is engaged. With the coupling disengaged, however, several problems arise in connection wth the use of the full OD input member. One of these is the "cold-start" condition which arises after the coupling has been inoperative for a period of time and fluid has leaked from the reservoir into the operating chamber, causing the coupling to operate as though it were engaged when it is intended to be disengaged. Upon start-up of the coupling under this condition, it typically takes a full minute or more for enough of the fluid to be discharged from the operating chamber back into the reservoir chamber to reduce the speed of the output member to its normal, disengaged level. During this period of time, operation of the coupling is normally not desired, e.g., the coupling is driving the radiator cooling fan of a vehicle engine and no cooling is required upon initial start-up of the vehicle engine. Moreover, the continued, engaged operation of the coupling for a period of several minutes, typically at speeds well above 1,000 rpm, results in an objectionable noise level, especially when the engine is warming up at fast idle. A related problem is the output speed level of the coupling in the disengaged condition. A relatively higher disengaged output speed (referred to as "idle speed") results in a relatively higher horsepower consumption by the coupling and the associated cooling fan with no resultant benefit.
As the need for improved fuel efficiency in automobiles has developed, production of smaller cars has increased and it has become more common to equip such cars with viscous fan drives which, because of their ability to be disengaged when engine cooling is not needed, greatly reduce overall horsepower consumption. One result of this trend has been greater interest in improving the disengaged operating characteristics of viscous fan drives, especially the idle speed, which tends to be higher in the smaller cars because the proportionately smaller fans can be driven at a relatively high speed and consume a substantial amount of input horsepower by a small amount of viscous fluid in the shear space. The reduction of idle speed toward the ultimate (i.e., the output speed resulting from bearing drag alone, with no fluid in the shear space) requires more complete pump-out of viscous fluid from the shear space. One aspect of maximizing pump-out is the ability to maintain high efficiency of the wiper which causes a build-up of pressure within the operating chamber, adjacent the discharge orifice, resulting in flow through the discharge orifice into the reservoir.
Those skilled in the art of viscous fan drives who have attempted to reduce idle speed recently have been following one general approach, i.e., minimizing the occurrence of parallel, closely spaced surfaces on the input and output coupling members around their peripheries or, where such parallel surfaces do exist, increasing the clearance between them. For example, in U.S. Pat. No. 3,990,556, the outer periphery of the input coupling member is provided with a series of notches such that adjacent notches join to form a line, rather than a surface and the specification of the cited patent states that "If there were provided any faces instead of a line of notch means ... the residual fluid in the working chamber would transmit the torque from the input member to the output member."
Similarly, in U.S. patent application Ser. No. 764,772, filed Feb. 2, 1977 in the name of K. R. Streeter, for a "FLUID COUPLING DEVICE WITH IMPROVED DISENGAGED OPERATING CHARACTERISTICS", assigned to the assignee of the present invention, the inner surface of the output member is cylindrical, while the outer surface of the input coupling member is frusto-conical, primarily to reduce the peripheral face-to-face engagement and reduce the idle speed.
As a further example, a commercially available viscous fan drive, produced by someone other than the assignee of the present invention, is basically of the full OD type discussed previously, but with the OD clearance (i.e., the radial dimension between the outer surface of the input member and the inner surface of the output member) increased to such an extent that, even though idle speed is reduced, the pump-out time is increased because of reduced wiper efficiency, as will be described in greater detail subsequently.